Japanese Patent Laying-Open No. 2004-303857 (PTD 1), for example, discloses, as a step-up transformer included in a DC-DC (direct current-direct current) converter which is a type of switching power supply, a structure in which several spiral turns of a primary-side coil on the high-voltage side and a turn of a secondary-side coil on the low-voltage side are stacked. Japanese Patent Laying-Open No. 2011-77328 (PTD 2), for example, discloses a structure in which secondary-side coils on the low-voltage side obtained by coupling two coils in series, each being wound a turn, into the S shape are arranged to hold a primary-side coil on the high-voltage side therebetween from above and below.